1. Field of the Invention
This invention relates to thin-film capacitors and, more particularly, to those serving as circuit components of ICs and LSIs. This invention further relates to a process of manufacturing these thin-film capacitors.
2. Description of The Related Art
With developing integrated-circuit techniques electronic circuits are becoming more and more miniaturized, and in turn the miniaturization of capacitors essential as circuit components for various electronic circuits has become a more important problem. Active components such as transistors are being miniaturized rapidly whereas miniaturization of thin-film capacitors comprising a dielectric thin-film layer, which are formed to operate together with active components on the same substrate, fall so behind that it is becoming a major inhibiting factor against making in more LSI or VLSI form. This is because of hitherto using, as material for dielectric thin-films, only materials having up to 10 at most in dielectric constant, such as SiO.sub.2 and Si.sub.3 N.sub.4. Therefore it is required for miniaturization of thin-film capacitors to develop dielectric thin-films having great dielectric constants. Of the ferroelectric oxides represented by chemical formula ABO.sub.3, perovskite oxides such as BaTiO.sub.3, SrTiO.sub.3 and PbZrO.sub.3, limenite oxides such as LiNbO.sub.3, or Bi.sub.4 Ti.sub.3 O.sub.12 are available not by themselves but also as solid solutions thereof, in the form of a single crystal or ceramic. These oxides are known to have dielectric constants ranging from 100 to as much as 10000, and are widely used for ceramic capacitors. Formation of thin-films from these materials is markedly effective for miniaturization of thin-film capacitors, and approaches to this have been carried out for a considerably long time. Particularly, examples resulting in relatively good characteristics are stated in a paper (Processing of the IEEE Vol. 59(10), 1440-1447) as follows: thin-films of BaTiO.sub.3 formed by sputtering and then heat treatment exhibited dielectric constants of 16 (when formed at room temperature) and 1900 (when formed by heat treatment at 1200.degree. C.).
For the formation of the conventional dielectric thin-films such as BaTiO.sub.3 mentioned above so as to have high dielectric constants, high temperature treatment is required. Such thin films can not be formed on the surface of the lower electrode, which is a structural element of the thin-film capacitor, unless the lower electrode is made from a noble metal having a high melting point, such as platinum or palladium. If the lower electrode is formed from a general electrode material such as aluminium, nichrome or copper, such electrode material might be subjected at high temperatures to evaporation and interaction with the dielectric film, resulting in a marked drop in the dielectric constant of the dielectric film. Even if made from a high melting point noble metal such as platinum or palladium, the formation of dielectric thin-films at 300.degree. C. or higher temperatures may bring the surface of the electrode into rough texture due to recrystallization. The dielectric film layer formed on such an electrode has nonuniform thickness, and therefore when voltage is applied, the smaller thickness is affected by the stronger electric field. This presents a problem with the dielectric breakdown characteristic.
Electrode material widely used for LSIs or VLSIs at present is a polycrystalline silicon or a low-resistance silicon layer formed by addition of an impurity at a high doping level to a part on a silicon substrate. These electrodes are collectively called "silicon electrodes" hereinafter. Silicon electrodes for which fine processing techniques have already been established are extensively used. Accordingly if a good thin-film of high dielectric constant could be formed on a silicon electrode, such a technique be applied to fabrication of IC capacitors. In the prior art, however, for example, a paper on SrTiO.sub.3 (IBM Journal of Research and Development, Nov. 1969, pp. 686-695) states on pages 687-688 that when a thin-film of high dielectric constant material is formed on a silicon electrode, an about 100 .ANG.-thick layer equivalent to silicon dioxide (SiO.sub.2) inevitably appears along the interface between them. Owing to its low dielectric constant, this interfacial layer contributes to a remarkable drop of the effective dielectric constant of the high dielectric constant film, and thus to make substantially invalid the advantage of using high dielectric material. Similar results are disclosed in another report of BaTiO.sub.3 (Journal on Vacuum Science and Technology Vol. 16(2), 315-318, refer to page 316).